Walsh & Hoyt: Trochlear (Fourth) Nerve Palsies

Trochlear nerve palsy is the most common cause of acquired vertical strabismus in the general population. A nuclear or fascicular lesion causes partial or complete paralysis of the contralateral superior oblique muscle, usually associated over time with overaction of its antagonist, the ipsilateral inferior oblique muscle. After issuing from the dorsum of the brain stem, the trochlear nerves cross almost immediately in the anterior medullary velum. Each nerve then runs in the subarachnoid space, through the cavernous sinus and superior orbital fissure, into the orbit, to the superior oblique muscle. Patients with a trochlear nerve palsy complain of vertical diplopia that is greatest in downgaze and gaze to the opposite side, because the superior oblique muscle is a depressor in adduction, when the muscle axis is parallel to the visual axis. The patients also have excyclotorsion when tested with double Maddox rods or Lancaster red-green glasses. The superior oblique muscle is an intorter, particularly when the eye is in abduction. The ability of the superior oblique muscle to intort the eye is particularly important when assessing superior oblique function in the presence of an oculomotor nerve palsy. The absence of intorsion when the patient attempts to depress the abducted eye indicates lack of superior oblique function as well.